Conversion of Xylose and Xylose-Phenol Mixtures to Hydrogen-Rich Gas by Supercritical Water in an Isothermal Microtube Flow Reactor

Microchannel reactors with inner diameters of less than 1.0 mm offer high rates of heat transfer to endothermic reforming reactions, particularly for the supercritical water gasification of biomass components such as hemicellulose and lignin, which are byproducts from cellulosic ethanol production. Xylose (the principal Sugar in hemicellulose) and phenol (the principal moiety in lignin) served as model compounds. The carbon in xylose was completely reformed to H-2-rich gas (62% H-2, 34% CO2) by supercritical water at 250 bar within a microtube flow reactor. Hydrogen gas yields of 8.2 +/- 0.6 mol H-2 mol(-1) xylose where achieved within a 1.0 s fluid residence time at 650 degrees C using a Hastelloy C276 microtube reactor, vs 750 degrees C using a 316 stainless steel microtube reactor. The short residence times were attributed to the high heat transfer rates and isothermal temperature profiles in the 508 mu m inner diameter microtube reactor, which were confirmed by modeling studies. Phenol was difficult to gasify. and the activation energy for phenol conversion was 264 +/- 20 kJ mol(-1). However, when a mixture of 1.6 mol phenol/mol xylose was gasified at 750 degrees C and 250 bar, the apparent rate constant of phenol conversion increased from 0.66 +/- 0.03 to 2.8 +/- 0.3 s(-1). Although the H-2 gas yield was 2.9 mol H-2 mol(-1) phenol + xylose (41% H-2 in product gas), if the 19% CH4 in the product gas was also reformed, the yield increased to 8.5 +/- 0.6 mol H-2 mol(-1) mixture.